A common type of flow sensor involves a bladed rotor of either the turbine type or the "paddle-wheel" type driven by the fluid such as water. The flow sensor is usually made as a unit, including a short length of pipe containing a rotor that is at the axis of the pipe, and a rotation sensing device is positioned close to the path of the rotor blades. One consideration limiting the use of flow sensors of that kind is that installed pipe must be cut in order to insert the flow sensor.
It has been found simpler to install a flow sensor of the probe type, having a rotor-and-sensor bearing tube inserted through a hole in the wall of the pipe. A technique is even available enabling the installation to be performed while the pipe carries liquid under pressure. Flow sensors of the probe type are also advantageous where the pipe diameter is large and where a standardized probe is to be used with various pipe diameters.
Similar considerations apply to flow sensors for air, and where the fluid conduit is round or rectangular duct rather than cylindrical pipe.
Rotor-type flow-sensing probes commonly use flow-straighteners to minimize error that would result from anomalies occurring in the flow such as a spiral spin of the fluid. However, if flow-straighteners are added to the rotor of a probe, the result would be an oversize probe, or one in which the size of the rotor is drastically reduced, for the probe to enter a normal-sized hole in a pipe. Miniaturized rotors have related problems of high cost, short life and impaired accuracy.